Thermoanalytical instruments, such as differential scanning calorimeters (DSC), are used to measure different characteristics and properties of a sample which is exposed to a temperature program.
A DSC is utilized to record temperature-related changes of the physical or chemical characteristics of a sample. These are for example heat measurements related to exothermic or endothermic events accompanying transitions and other effects occurring in a sample which is subjected to temperature changes. The changes of the sample are determined in relation to a reference, which can be an empty reference position or a suitable reference material. Depending on the type of DSC the reference or sample material can be placed directly on a respective measurement position or it can be placed in a suitable crucible, which is then placed on the measurement position.
Two main control principles for a DSC are well known, these are the heat flux principle and the power compensation principle. In the following, an example for a power compensated DSC will be discussed in more detail.
Power compensation is usually implemented into a thermoanalytical instrument by placing and separately controlling an additional heater, often referred to as compensation heater, at the sample position. The sample position, the reference position as well as any material placed on one of said positions are subjected to a temperature program, which is applied by the main heaters of the reference position and the sample position. The main heater of the sample position merely mimics the heating power delivered by the heater of the reference position. Said compensation heater is used to deliver any excess power needed for heating the sample in order to take it through endothermic phase transitions, while the temperature difference between the sample and the reference positions is controlled to remain substantially zero. Excess or compensation power is also needed for performing cooling runs, in which case the compensation heater applies a certain heat amount to the sample at the beginning of the experiment, which is gradually reduced during the experiment.
The reference position of a power compensated DSC is also equipped or in thermal contact with another compensation heater, which is set to a fixed offset voltage and provides a constant compensation power. In relation to said fixed reference offset the actual heating power demand of the sample expressed as the sample voltage can either be positive or negative. Such a power compensated DSC is for example disclosed in U.S. Pat. No. 6,632,015 B2 to Nagasawa.
For the analysis of very thin films and particles with masses in the microgram or even nanogram range different chip-based calorimeters were developed, which are often based on silicon technology. An overview over different uses of these chip calorimeters, such as e.g. high-speed DSC, is given by A. W. van Herwaarden “Overview of Calorimeter Chips for Various Applications”, Thermochimica Acta, 432 (2005), 192-201.
The realization of the power compensation principle for a chip-type DSC enhances several drawbacks of this principle, which were so far disregarded as being negligible. Depending on the setup, these drawbacks relate for example to the limited negative compensation headroom, the offset temperature as well as a baseline offset, drift and curvature.
Limited negative compensation headroom can lead to a cutoff of the measurable heat flow and can result in false or incomplete results. For power compensation the amount of ‘negative’ compensation power is limited by the offset compensation power set at the reference position. Therefore, the reference offset power has to be adapted in relation to the investigated sample. For an unknown sample it might even take several experimental runs to determine the appropriate reference offset power, which can result in wasting precious sample material as well as being time consuming.
Said reference offset power generates an offset temperature, which reduces the operational temperature range of the instrument. By reducing the reference offset power the offset temperature can also be reduced in order to expand the operational temperature range, but unfortunately this will increase the problem related to the limited negative compensation headroom. In order to ensure sufficient headroom e.g. for very fast cooling experiments, the resulting offset temperature can amount to several tens of degrees Celsius.
The baselines of the resulting DSC curves can also be offset due to the amount of offset power supplied at the reference position. Additionally, even when the offset voltage at the reference position is kept constant, the resulting offset power will vary with temperature as the resistance of the reference compensation heater depends on the temperature. This effect can result in an unwanted baseline drift and/or baseline curvature, which can further be superimposed by intrinsic physical differences between the sample and reference positions.
Therefore, the object of this invention is to provide a thermoanalytical instrument, in particular a differential scanning calorimeter (DSC), as well as a compensation principle for said thermoanalytical instrument, which overcomes the drawbacks of the power compensation principle.